A mixture formed of liquid photosensitive monomers has heretofore been used as an optical waveguide core layer-forming material intended for a mixed flexible printed circuit board for optical/electrical transmission, and in formation of the pattern of a core layer using the mixture, a desired core pattern has been produced by performing ultraviolet (UV) irradiation through a photomask. Such mixture formed of the photosensitive monomers has high photocuring sensitivity. On the other hand, when the mixture is subjected to a continuous process like a roll-to-roll (R-to-R) process serving as a production process bringing mass production into perspective from the viewpoint of surface stickiness (tackiness) after its application, a film formed of the mixture is broken upon its contact with a roll. Accordingly, the mixture has a drawback in that the mixture cannot be compatible with the R-to-R process, and hence involves a problem in that the mixture is poor in productivity (Patent Literature 1).
Accordingly, a polymer material that is solid at normal temperature has been generally used as a photosensitive resin in order to be compatible with a continuous process like the R-to-R process. At that time, as the molecular weight of the polymer material becomes higher, the flexibility of an amorphous film serving as a stage before its curing improves. On the other hand, the material involves a problem in that the patterning resolution (curing sensitivity) of the film reduces as the molecular weight becomes higher. In contrast, as the molecular weight of the polymer material becomes lower, the patterning resolution improves but the flexibility reduces. As described above, the flexibility and patterning resolution of the film are generally in a trade-off relationship, and the relationship has been perceived as a problem. In view of the foregoing, such an optical waveguide core layer-forming material that compatibility between the flexibility and patterning resolution of a photosensitive curable film is achieved has been required, and various kinds of such materials have been proposed (Patent Literature 2).
The optical waveguide core layer-forming material needs to satisfy many required characteristics, such as a high refractive index, high transparency, a high resolution patterning property, and high heat resistance, serving as various physical properties of a cured product in accordance with its use applications. Accordingly, investigations for satisfying the characteristics through, for example, the selection of various raw materials and blending balance have been made.
As described in the foregoing, in order that the optical waveguide core layer-forming material may be subjected to the R-to-R process bringing mass production into perspective, an approach involving turning an uncured film formed of the forming material into a dry film has been generally used. However, the approach leads to a reduction in degree of freedom in material design in material development owing to requirements for the process compatibility of the material for the dry film, such as the low tackiness and flexibility of the uncured product. In addition, when the dry film is produced, laminate base materials are needed for both surfaces of the film, and the need becomes a problem from the viewpoints of resource savings and a cost. Accordingly, in the material development, emphasis has also been placed on compatibility to a wet process and the compatibility has been investigated (Patent Literature 3).
In view of such background, there has been proposed, for example, a photosensitive resin composition satisfying the characteristics as a result of the blending of a special novolac-type polyfunctional epoxy resin serving as a main agent with various resins (Patent Literature 4).